IP MTU value set is greater than the current transport value 1306

[paul amaral]

I have an MTU issue I can’t figure out, on a Cisco 1100 when I configure a GRE tunnel and specify the MTU I get and fragmentation error

Int tun 650

ip mtu 1330

%Warning: IP MTU value set 1330 is greater than the current transport value 1306, fragmentation may occur

%TUN-4-MTUCONFIGEXCEEDSTRMTU_IPV4: Tunnel650 IPv4 MTU configured 1330 exceeds tunnel transport MTU 1306

Now when looking at the tunnel interface you can clearly see below that the transport MTU is set to 1476, which is the default MTU minus the 24-byte GRE header. Note that all I did was configure the tunnel interface without any other configuration, as soon as I set the MTU I get a warning.

So why am I getting a complaint about MTU fragmentation and where is this 1306 coming from? I should be able to set the MTU at 1330 since it’s below the transport MTU of 1476. I get the same error at 1400 MTU, it only goes away if its below 1306 and I have no idea where the router is getting 1306 from.

Could this be a cosmetic bug?

  #sh int tun 650 | in MTU

  MTU 9976 bytes, BW 100 Kbit/sec, DLY 50000 usec,

  Tunnel transport MTU 1476 bytes

Cisco IOS XE Software, Version 17.03.03

Cisco IOS Software [Amsterdam], ISR Software (ARMV8EL_LINUX_IOSD-UNIVERSALK9-M), Version 17.3.3, RELEASE SOFTWARE (fc7)

cisco C1161X-8PLTEP (1RU) processor with 1987967K/6147K bytes of memory.

[balaji.bandi]

what is the IOS code running ?

Both sides need to be the same  MTU setup ?. check the below output to see if you can find that anywhere configured?

show ip interface tunnel   --> show you  MTU for tunnel

show interface tunnel  ---> show you  MTU of physical interface 

show interface gix/x | include  MTU

show ip interface gix/x | include MTU

[paul amaral]

int tun 650

sh int tun 650

Tunnel transport MTU 1476 bytes

All other interfaces have the default MTU of 1500, as does tunnel 650 when created and shows 1476 which is -24 for GRE header.

I'm assuming this has to be a bug, When I create the tunnel and add the ip MTU right away I get a warning

#int tun 650

#ip mtu 1400
%Warning: IP MTU value set 1400 is greater than the current transport value 1306, fragmentation may occur

But if I create the tunnel, exit then come back and add the ip mtu I get no warning.

#no int tun 650

#int tun 650
#exit

#int tun 650
#ip mtu 1400

no warning!!

I searched on the ISO released caveats but didnt see anything that would cause this.

Cisco IOS XE Software, Version 17.03.03
Cisco IOS Software [Amsterdam], ISR Software (ARMV8EL_LINUX_IOSD-UNIVERSALK9-M), Version 17.3.3, RELEASE SOFTWARE (fc7)

Any ideas.

Paul